1. Field of the Invention
The present invention relates to radio frequency (RF) transponders such as radio frequency identification (RFID) transponders, and more specifically, to an RFID transponder adapted to a small form factor, such as a button or coin.
2. Description of Related Art
In the automatic data identification and collection industry, the use of RF transponders (also known as RF tags) has grown in prominence as a way to track data regarding an object to which an RF tag is affixed. An RFID tag is a type of RF tag that further includes a semiconductor memory in which information may be stored. An RFID interrogator containing a transmitter-receiver unit is used to query an RFID tag that may be at a distance from the interrogator. The RFID tag detects the interrogating signal and transmits a response signal containing encoded data back to the interrogator. RF and RFID systems are used in applications such as inventory management, security access, personnel identification, factory automation, automotive toll debiting, and vehicle identification, to name just a few.
These RFID systems provide certain advantages over conventional optical indicia recognition systems (e.g., bar code symbols). For example, the RFID tags may have a memory capacity of several kilobytes or more, which is substantially greater than the maximum amount of data that may be contained in a bar code symbol. The RFID tag memory may be re-written with new or additional data, which would not be possible with a printed bar code symbol. Moreover, RFID tags may be readable at a distance without requiring a direct line-of-sight view by the interrogator, unlike bar code symbols that must be within a direct line-of-sight and which may be entirely unreadable if the symbol is obscured or damaged. An additional advantage of RFID systems is that several transponders may be read by the interrogator at one time.
Since it is very desirable to include RF and RFID systems in compact applications, there is a critical need for RF and RFID tags having very small form factors, such as the size of a button or coin. Such small form factor RF transponders could be embedded in buttons for use in items of clothing, or in key chains, garment tags or parcel tags. A limiting factor in the fabrication of such an RF tag is the antenna. RF tags typically employ an antenna formed on a substrate and electrically connected to an integrated circuit. Various antenna configurations are known in the art, such as dipoles, meander dipoles, patches, etc. A typical dipole antenna has a physical length on the substrate of approximately one-half wavelength (.lambda./2) of the operative frequency of the transponder. For example, an RF tag that operates in the 2.45 GHz frequency range may have a dipole antenna approximately six centimeters long. Although this physical length is very small, it is nevertheless larger than a button or coin.
Accordingly, it would be very desirable to provide an RF transponder having a small form factor the size of a button or coin. More particularly, it would be very desirable to provide an RF transponder having an antenna covering a physical area that is smaller than approximately one-half wavelength (.lambda./2) of the operative frequency of the transponder.